1. Field of the Invention
The present invention relates to optical lens systems and, more particularly, to relay lens systems especially suitable for use in medical endoscopes and other optical imaging devices.
2. Description of Related Art
Medical endoscopes are used to view internal body organs through small openings created in and through the body wall or skin or by way of existing openings or orifices. Such instruments typically include a long, thin, rigid or semi-rigid optical cylinder that is mounted on or to a viewing mechanism or imager. When the endoscope is inserted and positioned for use, an image of the object being viewed is formed by an objective lens at the inserted or distal end of the endoscope. The image passes through a series of relay lenses along the cylinder to a viewer comprising an eyepiece or viewing lens or a video camera or imager at the viewing or proximal end of the endoscope.
The relay lenses must be very narrow and are typically in the range of about 2.4 mm to 6.0 mm in diameter and approximately 20 mm to 50 mm long. Each relay lens is commonly formed of two or more elements, and a pair of relay lenses make up a set of such lenses. Most endoscopes require two or more sets of relay lens pairs for proper operation. The number of sets generally depends on the length and specific requirements of the particular endoscope and/or the application for which it is intended.
Optimization of endoscope performance involves the adjustment, in the design and implementation of the endoscope, of many factors including brightness, contrast, resolution and corrections for various aberrations. Of particular significance are compensations for minimizing the basic Seidel aberrations (spherical aberration, coma, astigmatism, field curvature, distortion and axial and transversal chromatic aberration) as well as secondary spectrum and higher order aberrations. Typically, meaningful correction of one or more of these factors results in a concomitant loss of performance in one or more of the remaining factors.
Brightness of the image that is transmitted through a relay lens system is related to the ratio of the focal length and the diameter of the lens through which the image is being focused, a ratio commonly denoted the f-number. The smaller the f-number, the brighter the image transmitted by the lens. It is preferable in an endoscopic relay lens system to have a system f-number that is as small as possible so that maximum brightness is transmitted from the objective lens to the eyepiece. The resulting image brightness is also impacted by both absorption of energy by the lens media and unwanted reflection losses at interfaces of the lens elements.
Optical systems for medical endoscopes using relay lenses have been known since as early as 1879. In about 1908 a physicist from Zeiss, Moritz Rohr, developed improved optical systems for endoscopes using two or more relay systems. The introduction of anti-reflection coatings for optical components after the end of the Second World War allowed for the use of 3 or more relay systems in an endoscope.
In about 1959, H. H. Hopkins received a patent for a symmetric rod lens system. This relay system technology, and derivatives thereof, became the state of the art for most optical endoscope systems during the 1980""s and 1990""s. Although the Hopkins rod lens relay system corrects axial aberrations, it does not correct field curvature and astigmatism. Optical systems with that relay system necessarily require corrective elements in the objective system with extreme curvatures which are needed to compensate for the accumulated aberrations of the device""s multiple relay systems. The disadvantage of such systems is that for relay lenses with higher numerical apertures, the higher order aberrations of the objective system are unable to fully compensate for the higher order aberrations that are accumulated in the multiple relay systems of the device.
In the 1980""s and 1990""s efforts were made to develop relay systems for disposable endoscopes. See, for example, U.S. Pat. No. 4,946,267 to Hoogland, U.S. Pat. No. 4,784,118 to Fantone et al., and U.S. Pat. No. 5,188,092 to White. These relay systems use pressed aspheres formed of plastic material. Glass rods with plane surfaces may also be added to increase the numerical aperture. However, these relay lens systems do not attain the image quality achieved by systems using only glass lenses, and neither do they meet the performance expectations of surgeons. In addition, the costs per use tend to be higher, overall, than for endoscopes using non-disposable relay systems.
U.S. Pat. No. 5,059,009 to McKinley discloses an endoscope relay lens system containing a symmetric pair of rod lenses in which long flint glass rod lenses are cemented on opposite sides of a biconvex crown glass rod that is formed from a glass ball. This relay lens system, which forms an imaging system, corrects the off-axial aberrations better than relay lens systems based on the Hopkins patent, although the correction is not complete. Specifically, the meridional image curvature is overcorrected, so that the meridional and sagittal image curvatures bend with the same curvature on both sides of the Gaussian image plane but with different signs. As a result, astigmatism remains in this system. Furthermore, the meridional and sagittal image curvatures bend further away from one another as more relay systems are added. Thus, even if the Petzval sum is corrected using strong curvatures in the objective system, the over-corrected astigmatism of this relay system can only be compensated by an under-correction of astigmatism in the objective system. The consequence is a detrimental effect on correction of the coma.
The shortcomings of the McKinley lens system were partially addressed in U.S. Pat. No. 5,568,312 to Horton, in which the relay lens system comprises three axially aligned cylindrical lens pairs, with each lens pair having two substantially rod-like lenses. The system uses three different materialsxe2x80x94namely glass, cement and a polymerxe2x80x94as well as air interfaces to provide improved correction of chromatic and geometric aberrations. The effective f-number of each relay lens element is minimized, as is the number of lens element interfaces, to provide a brighter image. Problems, however, nevertheless remain in this system.
Relay systems currently available need to have both the axial aberration and off-axial aberrations corrected. The objective systems necessary to transfer the viewed object image to the entrance of the relay system and the eyepiece needed to transform the intermediate image from the end of the relay system to the image plane of the endoscope are only able to minimize the aberrations of their associated subsystem, and do not compensate for image aberrations that are accumulated in the relay system as a whole.
In addition, relay lenses for stereo endoscopes require a much higher numerical aperture than do relay lenses for mono endoscopes. Each of the two entrance pupils of a stereo endoscope must be as large as the single entrance pupil of a mono endoscope. But increasing the brightness of current stereo relay lens systems results in an unsatisfactory image. Currently available endoscope relay systems, when used in stereo applications, do not perform as well as mono endoscopes with such relay systems. Surgeons are therefore left in a disadvantaged position as the superior images produced in mono systems lack the apparent depth that can only be achieved with a stereo system.
There is currently a widespread and unsatisfied demand for better mono and stereo endoscopes. Surgeons using these instruments require that the image produced be both clear and sufficiently large and bright, with high resolution and low distortion. It is also desirable for there to be an appreciable three-dimensional or depth perception aspect to the image which requires a stereo relay system rather than a mono system. Finally, resolution in currently available endoscopic optical relay systems is rapidly becoming a limiting factor in the quality of the perceived image. New viewing technology such as HDTV camera sensors and imaging arrays, which allow for greater image resolution than heretofore available CCD (charge-coupled device) video chips, are becoming available for use in conjunction with endoscopes. This technology is driving a demand for endoscopes of increased image brightness, for endoscopes with higher magnification but the same resolution, and for endoscopes with higher resolutions than existing endoscopes. This strong demand for endoscopes with better overall performance can only be satisfied using relay systems of higher numerical aperture and better, or at least equivalent, correction of image aberrations than existing relay systems.
It is accordingly the desideratum of the present invention to provide a symmetric anastigmatic relay system for endoscopes which includes a relay lens system that corrects off-axial aberrations while exhibiting a higher numerical aperture than existing lens systems.
A symmetric anastigmatic relay system constructed in accordance with the present invention has a longitudinally and optically-aligned pair of meniscus (i.e. concavo-convex) shaped, elongated rod-type lenses formed of crown glass, a pair of meniscus-shaped flint lenses cemented on the convex end surfaces of the rod lenses, and a pair of two-element achromatic lenses positioned adjacent to the opposite-end concave end surfaces of the rod lenses. The achromatic lenses of the inventive arrangement satisfy the condition for xe2x80x9cnew achromats,xe2x80x9d namely nx/ny=vx/vy, where n is the refraction index and v is the Abbe number of the lens elements x and y that together form each achromatic lens. The achromatic lenses are mounted adjacent or in close proximity to the concave end faces of the rod lenses between the concave end faces and the intermediate images of the relay system so that they face, or confrontingly oppose, the intermediate images. The flint lenses cemented on the convex end faces of the rod lenses face the aperture of the relay system. The concave surfaces of the rod lenses, in conjunction with the achromatic lenses, reduce the field curvature of the relay system to zero. The cemented surfaces of the rod lenses and the cemented surfaces of the achromatic lenses reduce spherical aberration and astigmatism. Using flint and crown glasses in the rod lenses, in combination with the achromatic lenses, reduces primary chromatic aberrations to a minimum.
In an embodiment of the present invention, apochromatic glass combinations are used in the rod lenses to significantly reduce the secondary spectrum.
In a further embodiment of the invention for relay systems with high numerical aperture, the air surfaces of the flint lenses cemented to the rod lenses are aspheric, thereby providing a further reduction of spherical aberration.
In another embodiment of the invention, a reduction in variations of the correction of aberrations over or across the field of the intermediate images is achieved by making either the air-interfacing surfaces of the negative lens elements or the air-interfacing surfaces of the positive lens elements of the achromatic lenses aspheric.
In yet another embodiment of the invention, for relay systems with both high numerical aperture and large intermediate images, the air surfaces of the flint lenses cemented to the rod lenses and the air surfaces of either the negative lens elements or the positive lens elements of the achromatic lenses are aspheric.
In a still further and most preferred embodiment of the invention, an apochromatic glass combination is used in the rod lenses, aspheric surfaces are used on the air-interfacing surfaces of the flint lenses cemented to the rod lenses, and aspheric surfaces are used on the air-interfacing surfaces of either the negative lens elements or the positive lens elements of the achromatic lenses. Alternatively, instead of providing aspheric surfaces on the air-interfacing surfaces of the flint lenses cemented to the rod lenses, the glass for those flint lenses may be formed or manufactured from an axial gradient material. In another alternative, the glasses for either the negative lens elements or the positive lens elements of the achromatic lenses can be formed from axial gradient material.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.